Download Reducing Helium Costs more than 10

Reducing Helium Costs more than
10-fold with the PCT Gas Saver mode
Harry Prest
Technical Overview
This note describes a technique for minimizing helium carrier flow when the GC-MS
system is not in use. Helium has become expensive and costs of operation are
always a concern. Using the G1472A Rapid & Universal GC-MS Backflushing kit in
the Pressure Controlled Tee (PCT) configuration, total helium use can be lowered to
< 3 mL/min while the GC-MS is dormant. The MS analyzer is still under vacuum, at
operating temperatures and can be rapidly called into use. Total helium savings over
the course of a month or year are substantial using this mode.
Introduction
Recently the price of helium carrier for gas chromatography has dramatically
increased and supply has become limited. Many instrument users have become
concerned about saving this gas whenever possible. Saving helium carrier by cooling and venting GC-MS instruments (single quadrupole and triple quadrupole) is not
an efficient approach because air and water enter the vacuum manifold. Although
the instruments can rapidly reach analyzer operating temperatures, the removal of
chemisorbed water is a slow process; therefore the time necessary to reach stable
operation is lost analytical time. In addition, the frequent venting and pumpdown
cycles wear out the pump system components and tend to be time consuming. The
optimum situation has always been to keep an MS system under vacuum and at
temperature to allow the system background to continuously improve or at least
remain stable. This philosophy is also true for other GC detectors. Employing GC
instrument parameters to minimize gas use, such as standby methods using low
split ratios with low column flows and septum purge settings, risk air intrusion with
consequent damage to the inlet or column. These methods can produce a gas
usage rate of about 7 to 10 mL/min at best. While this is lower than the typical use
of greater than 24 mL/min during operations, it still represents a substantial
quantity of helium expended that is not available for analysis.
This note describes a state of the GC instrument enabled by the Pressure Controlled
Tee (PCT) and the Agilent Rapid Universal GC/MS Backflushing Kit (G1472A) in
which total helium carrier gas use is < 3 mL/min during the archived state of the
GC-MS instrument. This state is referred to as the “PCT Gas Saver mode” and,
while saving helium, the MS system remains under vacuum and at temperature and
rapidly can be brought into operation. In terms of helium carrier savings, nearly
every hour the instrument is placed in this PCT Gas Saver mode is equal to a saved
hour of analysis when compared to typical (default) operating conditions
(24 mL/min). Over the course of a year this can result in considerable helium gas
savings. For example, archiving a GC-MS instrument with the PCT Gas Saver
through every weekend in a year will result in a helium savings equivalent to more
than an additional 100 days or almost half of year of analytical operating time. This
technical overview contains specific directions for enabling this PCT Gas Saver
mode for PCT with the GC-MS. The approach can be easily generalized to other PCT
configurations to enable carrier gas savings on all GC systems.
Why the PCT for GC/MS Operation?
A simple schematic and an illustration of the PCT configuration are shown in
Figures 1 and 2. The analytical capillary column is split into two sections by a tee,
called the Purged Ultimate Union. Many configurations of the PCT are possible
because the column sections do not need to be of equal lengths. Using the PCT
improves GC-MS operation by providing rapid backflushing and rapid GC servicing.
Rapid backflushing removes late eluting matrix contaminants to:
• Avoid fouling the MS source thereby increasing instrument uptime and eliminating frequent source cleaning
• Avoid carryover, baseline rise, and compound retention time shifts over the
course of sample sequences
The PCT enables both post acquisition or Post Run backflushing and Concurrent
backflushing modes to minimize run time and cycle time. Concurrent backflushing
takes place while sample data acquisition is still underway.
Rapid servicing without venting the MS system allows:
• Quick capillary analytical column cutbacks to quickly restore chromatographic
performance
• Fast inlet maintenance including liner change and septum change
• Simple column servicing or the more efficient approach of guard or coated
precolumn exchange to restore compound chromatographic performance and
maintain compound retention times
PCT operation is simple to understand (Figure 1). During GC analysis, the pressure
applied at the Purged Ultimate Union (Pepc) is just a little higher than that needed to
prevent backflow into the electronic pneumatic control (EPC) module. After the GCMS analysis is completed, the pressure is raised at the tee (Purged Ultimate Union)
and lowered in the front section of column to push matrix out the split vent. This is
Post Run backflushing mode. In Concurrent backflushing mode, after the last analyte of interest has passed through the front column and the tee (Purged Ultimate
Union) and entered the second section of column, the head pressure at the inlet
can be dropped (Pinlet) so that the later eluting components will begin to retreat in
column 1. The last analytes then proceed to column 2 and enter the MS detector.
The MS system can be any of the Agilent systems including diffusion pumped systems.
Details and evidence of PCT performance are given in several application notes
[1-4] and instructions for the installation and operation are given in the Agilent
G1472A Rapid Universal GC/MS Backflushing Kit Pressure Controlled Tee manual
(G1472-90001).
For simplicity, this note describes work performed with the midpoint PCT configuration in which the Purged Ultimate Union is inserted between two 15-m columns
(0.25-mm id). The concept is easily generalized to other column arrangements.
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Injection Port
EPC
Vent
Pepc
Pinlet
Makeup line
Agilent
MS System
Purged Ultimate
Union (PUU)
Z+ mL/min
5975C MSD
7000A QQQ
Z mL/min
Column 2
Column 1
Agilent 7890A GC
Figure 1A. The PCT configuration schematically shown in the forward flow mode operating during analysis. The EPC device supplies just enough pressure to prevent backflow into the connecting
EPC device line (Pinlet> Pepc).
Injection Port
EPC
Vent
Pepc
Pinlet
Makeup line
Agilent
MS System
Purged Ultimate
Union (PUU)
X mL/min
5975C MSD
7000A QQQ
-x mL/min
Column 1
Column 2
Agilent 7890A GC
Figure 1B. The PCT configuration schematically shown in backflush mode. After completing the analysis
the EPC pressure has been raised to send flow back through the forward section of the column into the inlet and out the split vent (Pinlet< Pepc).
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Figure 2.
Picture of an installed PCT: Column 1 connects (dash-dot-dash) the injection port to the
Purged Ultimate Union (at right) and Column 2 connects (dash) the MS system to the Purged
Ultimate Union.
Procedure for PCT Gas Saver Mode
To put the system in PCT Gas Saver mode:
1. Set the injection port temperature to OFF and allow it to cool to room
temperature. The port will cool faster with the GC oven set to 20 °C.
After the oven has reached room temperature it can be turned OFF to save
power.
2. Set the Column 2 flow to 1.5 mL/min (constant flow) (Figure 3).
(Note: for columns narrower than the 0.25 mm id used in this example,
set the pressure at Column 2 to 3 psi. For example, a 10 m × 0.18 mm id
Column 2 would only require a flow of ~0.6 mL/min which results in even
higher gas savings.)
Figure 3.
Column 2 (AUX → MSD) conditions: column flow setpoint 1.5 mL/min.
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3. When the injection port and oven have cooled, set the inlet pressure to OFF. The
port must then be opened by either removing the septum and septum nut or
rotating the twist-top to release the liner connection (Figure 4). This releases
the pressure inside the port and the inlet. The Column 1 display should read zero
or close to it.
(Alternatively, if the user does not want to open the injection port, then:
set the 7890 Gas Saver to OFF, the inlet pressure to 0, total flow to 0 and septum
purge to 0. The 7890 system will beep as it can not achieve the setpoints.)
Figure 4.
When the injection port is cold, loosen the injection twist-top by counter-clockwise rotation
of the yellow handle. Make sure to lift the top slightly to insure the seal is broken.
4. In this 15 m × 15 m column configuration, the calculated flow displayed on the
GC panel for Column 1 near the inlet should be negative and approximately
−0.53 mL/min (Figure 5). This shows column flow is back from the Purged
Ultimate Union into the inlet.
Figure 5.
Column 1 (Inlet → Aux) conditions: pressure OFF, the flow calculation result is shown and
the negative sign indicates flow is back into the injection port.
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To return the system back to use:
1. Set the Column 2 flow to 3 mL/min, for diffusion pump systems, or 8 mL/min for
turbo systems. See the PCT User’s Manual
2. Reseal the injection port and set the inlet pressure to 2 psi.
3. Set the septum purge to ON at ≥3 mL/min and gas saver on with a split flow of
≥20 mL/min.
4. Turn on the injection port temperature.
The Column 1 flow will be negative (−1.7 mL/min in 15 m × 0.25 mm id configuration) and as the port warms up to operating temperature, contaminants accumulated during the idle period will be backflushed out the split vent and not transferred
into the analytical column.
Results
Comparing the air/water background in the mass spectrometer reveals no measureable increase due to air intrusion between the operating state and this PCT Gas
Saver mode even when compared over a > 72-hour weekend period (Figure 6).
Total gas use is about 2.6 mL/min for this configuration. The default 7890 Gas Saver
mode is a split flow of 20 mL/min, an additional flow used by the septum purge is
3 mL/min (minimum), and the column flow of about 1.2 mL/min means that the
minimum use in operation is 24 mL/min. If the user were to apply this PCT Gas
Saver mode over only weekends (from Friday at 6 pm to 8 am Monday morning,
62 hours), they would gain about 55 hours of operation every week. If this is applied
every weekend of a year, the helium savings is over 2600 hours or 100 days of operation each year. Even overnight from 6 pm to 8 am saves helium carrier equivalent
to ~12.5 hours of operating time or more than an additional 8-hour day of operation.
Also worth noting is that PCT Gas Saver mode, unlike 7890 Gas Saver mode, causes
a very small backward flow through the port. Therefore, trace carrier contaminants
do not accumulate as rapidly as they would at higher split flow. This is similar to the
PCT Standby mode where the system is in backflush and column is kept clean [5].
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Figure 6.
MS background with 1.5-mL/min helium carrier and no flow contribution from front split/
splitless injection port. Upper panel at time zero and lower panel at 72 hours later. No change
in the 18, 28 or 32 background is detected over the period. (Note: a small air leak does exist in
this system).
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Conclusions
The PCT configuration provides a number of features that improve GC-MS operation.
These include rapid backflushing to protect the column and MS source and increase
productivity, and quick, ventless servicing of the column and inlet to maintain data
quality. The PCT Gas Saver mode is an additional capability which “archives” the
GC-MS system with extremely low helium carrier gas use. This allows the MS analyzer to remain at temperatures where the background and stability of the system
improves, or maintains its integrity. The GC system can be quickly called into use so
there is less time required than in venting the system and returning it to service.
PCT Gas Saving Mode uses carrier at < 3 mL/min. When compared to typical GCMS use, every hour the system is in PCT Gas Saver Mode is an additional hour of
analytical time and carrier saved. Routinely archiving the system in this mode for
every weekend of a year equates to more than 100 days of analytical time recovered.
Since the PCT can be enabled on any GC-MS (or GC) system, an entire laboratory
can employ these savings to dramatically reduce one of the major costs of operation. Table 1 shows a summary of the additional analytical time that can be provided
by PCT Gas Saver mode over various durations.
Table 1.
Additional Operating Analytical Time (Days) Supplied by the PCT Gas Saver Mode as a
Function of the Duration Applied and Period of Consistent Use.
PCT Gas Saver Mode Use
Period
Every night
Only weekends
Evening and weekends
1 Week
1 Month
1 Year
3.6 days
14.6 days
174.6 days
2.3 days
9.2 days
110.5 days
4.4 days
17.5 days
210.3 days
Note: For example, using PCT Gas Saver Mode only on the weekends will add an additional 110 days/year of operation
compared to the default Gas Savers Settings of 20 mL/min (plus column and septum flows of 1.2 mL/min and
3 mL/min, respectively).
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References
1. “Capillary Flow Technology for GC/MS: A Simple Tee Configuration for Analysis
at Trace Concentrations with Rapid Backflushing for Matrix Elimination,”
Agilent Technologies publication (5989-8664EN)
2. “Capillary Flow Technology for GC/MS: Efficacy of the Simple Tee Configuration
for Robust Analysis Using Rapid Backflushing for Matrix Elimination,” Agilent
Technologies publication (5989-9359EN)
3. “Maintaining Compound Retention Times with the Backflush enabled Pressure
Controlled Tee Configuration for Agilent 7890A GCs with Agilent 5975 Series
MSD and Agilent 7000 Series Triple Quadrupole MS Systems,” Agilent
Technologies publication (5990-4643EN)
4. “Implementation of the Pressure Controlled Tee for Backflushing for the 7000
Series Triple Quadrupole Mass Spectrometer: Implications for Sensitivity,”
Agilent Technologies publication (5990-4504EN)
5. “User Quick Guide to Pressure Controlled Tee (PCT) Operation - Post Run
Backflushing,” Agilent Technologies publication (5990-5484EN)
Acknowledgements
The author is grateful for many helpful discussions with Bruce Quimby and
Bob Henderson at Agilent Technologies, Wilmington, DE.
For More Information
For more information on our products and services, visit our Web site at
www.agilent.com/chem.
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© Agilent Technologies, Inc., 2010
Printed in the USA
April 1, 2010
5990-5444EN